The treatment of herpes labialis, the clinical manifestation-of- oral-facial infections by type 1 and to a limited extent type 2 herpes simplex viruses (HSV-1 and -2), continues to be a major concern of dentists and physicians. Because of the localized, cutaneous nature of the disease, topical drug administration is the ideal treatment route. Although a few antiviral drugs currently are available, they are not useful in the therapy of herpes labialis due to problems of systemic drug distribution and/or topical drug bioavailability. Thiosemicarbazones (TSCs) are a class of compounds active as inhibitors of ribonucleotide reductase. A form of this enzyme is specified by HSV-1 and HSV-2 and therefore should be a target for antiviral drugs. Certain TSCs are partially selective inhibitors of the viral enzyme and are active against HSV-1 by topical application in an animal model. The objective of this project, therefore, is to identify TSC derivatives which are much more potent and selective inhibitors of HSV-1-specified ribonucleotide reductase and to evaluate promising compounds for antiviral efficacy in vitro. These objectives will be accomplished through a series of studies which will determine structure-activity relationships among a large group of TSCs as inhibitors of the viral reductase. Lead compounds for this study initially will be chosen on predictions outlined in Preliminary Studies. Activities against the viral enzyme will be compared to activities against the enzyme from uninfected mammalian cells. TSC analogs found to be both potent and selective inhibitors of the HSV reductase will be assayed in vitro for their capacity to inhibit HSV-1 replication. Active compounds also will be assayed in vitro for cytotoxicity. In vitro therapeutic indexes will be computed to determine the degree of antiviral selectivity and will be compared to enzyme selectivity indexes for possible correlation. Finally, selected compounds will be studied to more clearly elucidate the mechanisms by which ribonucleotide reductases are inhibited and the in vitro consequences of this inhibition. The mechanism studies will (i) determine if enzyme inactivation occurs, (ii) examine whether enzyme inactivation leads to irrevers- ible inhibition of virus replication. (iii) determine stability constants for the complexation of TSCs with metals and compare such constants to the inhibitory constants of the reductases, and (iv) quantitate deoxynucleotide pool sizes in HSV-1-infected cells to correlate enzyme inhibition with pool size changes.